One of the challenges in semiconductor device processing is the problem of metal contamination on the substrate surface. Semiconductor devices are extremely sensitive to minute quantities of metals present on the surface or inside the substrates. These metallic species are well known to deteriorate the electrical performance of the active layer. The silicon substrate industry is very mature and major suppliers are able to provide material with very low levels of metal contamination (at the surface and in bulk). The sapphire (Al2O3 single crystal) industry, one of the most important metal oxides with wide applications in the semiconductor industry, is less mature, even though it has been improving in recent years. Particularly, sapphire wafers are used for Silicon-on-Sapphire (hereafter “SOS”) fabrication, but also as substrates for deposition of III-nitride films for LED devices. Due to a lower manufacturing maturity, sapphire wafers can be contaminated at different steps of substrate preparation and handling processes like polishing, storage and characterization. A large quantity of metal contamination can often be found on the surface of a sapphire substrate used for semiconductor devices. The metal contamination mainly comprises Ti, Fe and Ni elements, which are typically present in the slurry used to polish the sapphire surface.
In the fabrication processes of SOS substrates, a thin active silicon layer is grown or transferred onto a sapphire substrate. The high temperature thermal treatments carried out during or after the fabrication process of SOS substrates trigger the migration of the metal species from the surface of the sapphire substrate toward the silicon layer, thus rendering the SOS substrates incompatible with the manufacturing of electronic devices. Particularly, the diffusion velocity of the iron in the silicon is high and its solubility in silicon is very low. Under the condition of significant iron contamination, iron silicide precipitates can form in the thin active silicon layer and degrade its electrical integrity.
The RCA cleaning sequence is a standard method for cleaning a silicon surface. However, a sapphire substrate is more chemically inert than silicon, and requires more aggressive chemical treatments for etching its surface and dissolving metallic contaminants.
Chinese Patent CN102218410 recites a method for cleaning the surface of a polished sapphire substrate comprising successive wet treatments based on basic and acidic solutions, including ammonia peroxide mixtures, sulfuric peroxide mixtures and HF mixtures. It teaches that organic and inorganic compounds and metal ions can be effectively removed.
In Chinese Patent CN103537453, the invention relates to a method for cleaning a polished sapphire substrate based on ultrasonic baths, able to remove particles, metal ions and organics compounds.
In Chinese Patent CN101912855, the invention relates to a water polishing solution prepared from neutral media. By carrying out water polishing with the abovementioned solution immediately after alkaline polishing is finished, the residual CMP polishing solution can be flushed away, easily-cleaned materials can be adsorbed, the surface tension can be quickly reduced, thereby obtaining a clean polished surface.
In another document, Dan Zhang and Yang Gan make a general review of wet and dry cleaning processes applied to sapphire single crystal substrates (“Recent Progress on Critical Cleaning of Sapphire Single Crystal Substrates: A Mini-Review”—Recent Patents on Chemical Engineering, 2013, vol. 6, (pp. 161-166). Among different cleaning processes, they review, in particular, UV and plasma irradiation able to remove thin layers of organic contaminants, but with most inorganic contaminants remaining unaffected. They also note a change in surface morphology after this type of treatment.
The metal contamination elements can be located at the surface and in the sub-surface of substrates, from 0.5 nm to 1.5 nm deep, for example, and possibly up to 5.0 nm deep. This is usually why wet treatments reacting only with the surface of the substrate are not sufficiently efficient and do not meet the specific requirements for the manufacturing of electronic devices. Additionally, for substrates with low reactivity to chemicals, such as sapphire, wet treatments are inefficient to dissolve all metal contaminant elements, even at the surface of the substrate, depending on the chemical bonds of those elements.
Chemical-mechanical polishing can eventually remove the sub-surface contaminated layer, but it remains difficult to prevent slurry contaminant residues and their slight diffusion at the close surface, and it is difficult or impossible to reach the targeted levels of residual metal contaminants.
Alternatively, a wet cleaning step based on phosphoric acid (H3PO4) chemistry and performed at high temperature (100° C. or higher) can be efficient to remove metal contamination at the surface and close sub-surface; nevertheless, this type of solution is difficult to implement in an industrial infrastructure due to safety and the degraded reliability of bath heater elements at such high temperatures. Moreover, a residual contamination of phosphorus has been observed at the surface after such treatments, which may be unacceptable for certain targeted electronics applications given that phosphorus is a dopant for silicon.
The wet or dry chemical etch solutions currently existing are either inefficient to remove metal contamination at the surface of substrates having low reactivity (such as sapphire) or, when efficient, at least partially, they may degrade the substrate surface (local surface pitting, increased roughness, etc.) to an extent that the substrate is not suitable for further use.
In view of the above, a first technical objective of the present disclosure is to develop a more efficient metallic contaminant cleaning method.
Another technical objective of the present disclosure is to provide a method, which limits surface degradation, in order to be compatible with a direct molecular adhesion process in which one substrate is directly bonded onto another substrate along the surface previously treated by the metallic contaminant cleaning method.